1. Field of the Invention
The present invention relates generally to an information recording system for recording/reproducing information on/from an information recording medium having recording tracks, and more particularly to an information recording system in which a velocity control process is performed to position a head onto an appointed track in accordance with optimal reference velocity information.
2. Description of the Related Art
Information processing apparatuses such as optical disk apparatuses have conventionally been used, in which a laser beam emitted from a laser diode mounted on an optical head is used to record information on an optical disk (information recording medium) having recording tracks or to reproduce information recorded on the optical disk.
In this type of optical disk apparatus, the optical head is moved in a seek operation by a linear motor (or an actuator) in the radial direction of the optical disk, i.e. in a direction crossing the tracks, so that the radiation point of a laser beam is shifted to a appointed track.
In this seek operation of the optical head, when the optical head has approached the appointed track, the velocity of the optical head is controlled by a reference velocity signal for deceleration. The reference velocity signal is generated so that a pull in to the tracking operation is performed at the time the optical head has reached the appointed track. On the basis of the reference velocity signal, the optical disk apparatus controls the velocity of the optical head within a range of velocity which depends on the system of the optical disk apparatus and at which the pull in to the tracking control can be executed. In this optical disk apparatus, the velocity of the optical disk is controlled within the range of velocity, which depends on the apparatus and at which the tracking control can be executed, in accordance with the predetermined reference velocity signal (see Jap. Pat. Appln. KOKAI Publication No. (Hei)1-271921). FIG. 1 is a graph showing an example of the reference velocity for controlling the velocity of the optical head. As shown in FIG. 1, the velocity of the optical head is designed to decrease at a predetermined rate as the optical head approaches the appointed track. The designed value of the velocity is set as a reference velocity. The reference velocity is set to decrease the velocity of the optical head so that the pull in the tracking operation can be executed at the time the optical head has reached the appointed track.
However, in the optical disk apparatus in which the above-mentioned velocity control is performed, the control range of the velocity control system is limited, and there occurs a deviation of velocity due to a velocity control system, as shown in FIG. 2. FIG. 2 shows an example of the velocity control system applied to the optical disk apparatus. In FIG. 2, an optical disk 90 and an optical head 91 are shown schematically. A signal processing unit 92 receives velocity deviation data, position deviation data, etc. and converts these data items in a predetermined data format. On the basis of the converted data output from the signal processing unit 92, a velocity detector 93 detects the velocity of the optical head, and a number-of-remaining-tracks calculator 94 calculates the number of tracks remaining between the track at which the head is located at present and the appointed track. A reference velocity table 95 reads out a designed reference velocity associated with the calculated number of remaining tracks output from the number-of-remaining-tracks calculator 94. A gain controller/power amplifier 96 controls the velocity of the optical head 91 on the basis of the velocity of the optical head 91 detected by the velocity detector 93 and the designed reference velocity read out from the reference velocity table 95. In this velocity control system, a velocity deviation depending on the system will occur, as mentioned above.
FIG. 3 is a graph showing a designed reference velocity 97 at the time of deceleration, an actual movement velocity 98 of the optical head, and a velocity deviation 99. In the conventional optical disk apparatus, when the tracking control is executed in accordance with the reference velocity, the velocity of the optical head does not sufficiently decrease because of the velocity deviation, and consequently the pull in to the tracking control may fail. In such a case, the optical disk apparatus needs to read out the current position data (address) of the optical head and restart the operation of making the head seek the appointed track, or to restart the operation of making the head seek the appointed track after returning the head to a predetermined position. The failure of the pull in to the tracking control will increase the time of movement (seek time) of the head to the appointed track and will decrease the efficiency of the information processing of the information processing apparatus.
In another type of optical disk apparatus, when the reference velocity set at the design time has decreased to a predetermined value, the apparatus waits for a predetermined time period until the actual velocity of the optical head falls within the range in which the pull in to the tracking control can be performed. After the wait, the pull in to the tracking control is performed. In this apparatus, however, the wait time is required until the actual velocity of the head falls within the range in which the pull in to the tracking control can be performed, and the seek time increases.